The present invention relates to a duplex reproducing apparatus and, more particularly, to a duplex reproducing apparatus operating on an electro-photography principle. The present invention relates to a duplex reproducing apparatus preferable for printing both faces of a recording sheet comprising a cut paper.
A conventional duplex reproducing apparatus is disclosed in, for example, U.S. Pat. No. 4,588,281. Toner is used for developing images in this kind of duplex reproducing apparatus. FIG. 9 is a sectional side view typifying the conventional duplex reproducing apparatus.
The conventional duplex reproducing apparatus comprises a heat roll 1, a back-up roll 2, an image forming station which includes a photo sensitive drum 3, and a forward recording sheet carrying passage 4. A fixing station comprises mainly the heat roll 1 and the back-up roll 2.
The duplex reproducing apparatus comprises further an intermediate recording sheet carrying passage 5, an inversion recording sheet carrying passage 6, a backward recording sheet carrying passage 7 and a discharge recording sheet carrying passage 8. These recording sheet carrying passages 4, 5, 6, 7 and 8 are constituted mainly of a belt drive conveyor and a jam preventing iron plate which is provided at a seam between belt drive conveyors.
The duplex reproducing apparatus comprises an inversion claw 9, a recording sheet inlet 10, an inversion tray 11, and a discharge recording sheet stacking means 12.
A recording sheet 13 fed into the duplex reproducing apparatus from the recording sheet inlet 10 is conveyed to a an image forming station, where a toner image 14 is generated. The toner image 14 is present in this case on a lower surface of the recording sheet 13. Next, the recording sheet 13 is carried to the fixing station constituted mainly of the heat roll 1 and the back-up roll 2.
The heat roll 1 has a heater incorporated therein, which works as a heat source for heating the recording sheet 13. Then, while the back-up roll 2 is not provided with a heater, it is warmed up to a temperature high enough to fuse the toner image 14 as it presses functionally the recording sheet 13 being heated by the heat roll 1.
The toner generally does not have a definite fusing point and a definite solidifying point, however, the present invention refers to a temperature whereat the toner image 14 is capable of being transferred to other substances.
The recording sheet 13 is present under the forward recording sheet carrying passage 4 which is provided between the an image forming station and the fixing station (heat roll 1 and back-up roll 2), however, the recording sheet 13 is carried to the forward recording sheet carrying passage 4 on a suction of the air current. The recording sheet carrying method also applies likewise to the inversion recording sheet carrying passage 6 which will be described herein-later.
The recording sheet 13 having reached the fixing station is heated by the heat roll 1, thus the toner image 14 present on the lower surface of the recording sheet 13 is fused and then fixed on the recording sheet 13.
Next, the recording sheet 13 is carried to the inversion claw 9 through the intermediate recording sheet carrying passage 5. In this case the inversion claw 9 closes, if the recording sheet 13 necessary to have both sides printed, immediately before the recording sheet 13 reaches the inversion claw 9 with the one side printed. FIG. 9 shows the state where the inversion claw 9 has closed.
In the present invention the side of the recording sheet 13 which is printed first is called a first face, and the back side of the recording sheet 13 is called a second face. Upon closing of the inversion claw 9, the recording sheet 13 is fed into the inversion recording sheet carrying passage 6 and carried to the inversion tray 11. The first face of the recording sheet 13 comes upward in the inversion tray 11 according to the operation mentioned above.
Next, the recording sheet 13 is allowed into the backward recording sheet carrying passage 7, gets to the an image forming station by way of the backward recording sheet carrying passage 7 and the forward recording sheet carrying passage 4. The toner image 14 is thus generated on the second face of the recording sheet 13, and the then it is carried immediately before the inversion claw 9 as in the case of the first face fixing of the recording sheet 13.
In this case, the inversion claw 9 opens upward, and the recording sheet 13 having both face sides printed is discharged to the discharge recording sheet stacking means 12 by way of the discharge recording sheet carrying passage 8.
According to the series of operations described above, the toner image 14 is formed on both the first face and second face of the recording sheet 13 and then is fixed on the recording sheet 13.
Thus, the above stated construction and operation of the conventional duplex reproducing apparatus have been described with reference to the movement of one recording sheet 13. The recording sheet 13 used here is a cut paper. Further, the operation of the duplex reproducing apparatus is carried out sequentially.
A stain on parts such as the recording sheet carrying passages 4, 5, 6 and 7, the inversion claw 9 and others is not taken into consideration in the above described conventional duplex reproducing apparatus. The toner which is heated and fused by the fixing station is not cooled down thoroughly and comes in contact with the parts such as the recording sheet carrying passages 4, 5, 6 and 7, the inversion claw 9 and others as fused, thus staining the recording sheet carrying passages 4, 5, 6 and 7.
Consequently, in the conventional duplex reproducing apparatus shown in FIG. 9, a known method is such that the recording sheet 13 is carried on a lower side of the intermediate recording sheet carrying passage 5, and the face of the recording sheet 13 in contact with the back-up roll 2 is brought into contact with the intermediate recording sheet carrying passage 5.
However, the back-up roll 2 is also heated by the heat roll 1 up to a temperature high enough to fuse the toner. Thus, the toner image 14 on the first face of the recording sheet, 13 is refused by the back-up roll 2 at the time of the second face fixing of the recording sheet 13.
Therefore, even when employing such a recording sheet carrying method, the toner comes in contact with the recording sheet carrying passages 4, 5, 6 and 7 as fused and stains the recording sheet carrying passages 4, 5, 6 and 7, which is problematical.
Besides, the above problem may be present even in the case of a fixing method (flash fixing method, for example) other than that for which a heat roll is used, so far as the fixing method on heating is employed.
In case, for example, a long continuous paper is used like rotary printing, the above mentioned problem will be solved by retaining the continuous paper with tension, however, the problem is still not soluble in the conventional duplex reproducing apparatus, so far as cut paper is used as the recording sheet.
The problem of stain on the parts such as recording sheet carrying passage members and others in the cut paper printing is particularly influential in a duplex reproducing apparatus having high printing speed, because the higher the recording sheet carrying speed is, the longer the toner image is carried as fused.